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1999 15 hp Mercury runs good but has Loss of power.

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  • 1999 15 hp Mercury runs good but has Loss of power.

    I have a 1999 15 hp Mercury outboard. It starts first pull, ideals great. It pushes a 10 ft. rib inflatable to 25 mph. However, it will no longer put more then two people on plain. When it was new and for the first 10 years or so it would put four people on plain. What has caused the power loss? It has great speed but not enough power to put more then two people on plain. Over the years it has been run at 1\4 to 1/3 throttle while mounted to a 30 ft Sea Ray (trolling) and 3/4 to full throttle while mounted on the inflatable. My estimate of time run each summer is 15 hours, half while mounted on the big boat and half while mounted on the inflatable.
    I have checked the compression and it is some what low. Each cylinder test to the same level. Can I put new rings in it without changing the power head? Any other ideas?
    Thanks for your help.

    Wayne
    Last edited by The Miss Emilie; April 25th, 2014, 08:02 AM.

  • #2
    Re: 1999 15 hp Mercury runs good but has Loss of power.

    Is there spark on both cylinders ?----------I believe you can inspect pistons and rings with exhaust cover off.--------What are the compression values and is the gauge known to read properly ?---Could be a reed valve issue as well.-----Caburetor flooding ?------Fuel pump diaphragm ?-----Lower crankcase seal ?--Yes you can install new rings.----Best to find what the real problem is !

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    • #3
      Re: 1999 15 hp Mercury runs good but has Loss of power.

      Yes there is spark on both cylinders. I check the compression last summer and don't remember the number other then it was low but even on both cylinders. When checking the compression, I followed the proper procedures for checking compression. I have to assume the gauge was correct. I think that if the reed valve was bad or the carburetor was flooding or fuel pump was bad the motor would not be starting easily, running smoothly and reaching speeds of 25 mph. All of those things don't seem to be an issue. I had the crankcase seal replaced last summer. I am betting that the problem is compression. I am wondering, do others agree and can the problem be corrected without replacing the power head, which cost well over $1,000? One thing I did forget to mention is that the engine is a two stroke.

      Thank you for your feed back and thoughts.

      Wayne
      Last edited by The Miss Emilie; April 25th, 2014, 08:27 AM.

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      • #4
        Re: 1999 15 hp Mercury runs good but has Loss of power.

        Recheck the compression and report back. it cost nothing to do that.

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        • #5
          Re: 1999 15 hp Mercury runs good but has Loss of power.

          Clean carb, proper adjustment, WOT Ignition timing, and the correct carb/throttle linakge sync are critical to performance.

          WOT RPM is very important in deteermining correct prop pitch. Swapping that motor from one craft to another may need a different prop. A higher pitch prop on the lighter craft may not be a good selection for the other craft and over-load the motor, causing accelerated wear and tear on the motor.

          Test compression with a couple different compression testers, their reads can vary widely. The fact that your reads are quite similar as good. Many motors will continue to provide good performance with compression in the low 100's on an accurate gauge. If the motor starts easily and idles well, compression is generally not the issue.

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          • #6
            Re: 1999 15 hp Mercury runs good but has Loss of power.

            I would also check the pitch or setting of your outboard. Not being in correct position will diminish your performance.

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            • #7
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              Was the problem fixed? If so what was it? I have a 95 8hp that is doing the same thing and I have had the carb rebuilt, new coil packs, plugs, wires and checked compression. Compression was 10lbs low in each cyl. The motor runs amazing but very slow and won't plain out a 12' aluminum boat that it has been on since 1995

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